Thanks for writing in!
Can you let us know the WPMU plugin that you are using as they have bunch of plugins on offering? Also, please note that 3rd party plugin conflict falls outside the scope of support we offer and that needs to be directed at the plugin developer. However, I would like to take a look and see if some solution can be provided.
Please share the website login details in secure note. In the meantime please make sure to increase memory limit and se how it goes. You can refer following resource to increase memory limit: